ABSTRACT The importance of tissue resident macrophages for pathogen clearance and homeostasis is beginning to emerge. However, our understanding of the multifaceted roles these macrophages play in mucosal tissues such as the lung remains incomplete. The lung is a very complex organ with specialized structures to allow for adequate gas exchange. The pulmonary microenvironment is unique and has a direct and important influence on the resident immune cells, especially macrophage populations. Currently, it is well established that lung harbors two distinct populations of macrophages known as alveolar macrophages (AMs) and interstitial macrophages (IMs). One of the most important function of pulmonary macrophages is to regulate inflammatory pathways during infection and allow for the resolution inflammation after the pathogen is cleared. The precise mechanisms that macrophage populations utilize to accomplish these critical functions in vivo are not well understood. For example, Macrophages play a critical role in regulating pathogen induced inflammation during a respiratory infection such as with influenza. Influenza infection causes worldwide yearly epidemics resulting in thousands of deaths and hospitalizations. Clinical complications are caused by tissue damage due to excessive viral-induced immune responses. Thus, there is a critical need to understand the cellular and molecular mechanisms that cause infection-induced inflammation and pathology in vivo in order to develop new therapeutic strategies to alleviate damaging inflammation during infection. Here we report a previously uncharacterized subset of pulmonary macrophages that are exclusively localized around the large bronchiole airways. We have termed these cells as large airway associated interstitial macrophages (LAAMs). Our preliminary data show that the transcriptional gene signature of the LAAMs is remarkably unique when compared to AMs with high expression of regulatory genes and thus, we believe that these cells play an important role in regulating influenza-induced inflammation in vivo. Furthermore, the unique and exclusive localization of LAAMs to the large airways suggests that these macrophages may serve a ?gatekeeper? function within the complex structure of the lung. Thus, we hypothesize that LAAMs define a novel population of macrophages completely distinct from other pulmonary macrophages such as AMs, and their unique positioning, remarkable gene expression profile, and the notable reaction to influenza infection make them critically important for regulating immune and tissue homeostasis and pathogen-induced inflammation. We propose the following aims: Aim1: To determine the ontogeny, maintenance and cellular heterogeneity of LAAMs; Aim2: To determine the physiological significance of LAAMs and AMs following infection; Aim3: To determine the mechanisms, that allow LAAMs to regulate immune/tissue homeostasis in steady-state conditions or during respiratory infection.